Guam's forest avifauna has declined sharply while avian populations remained relatively stable on other nearby islands. This study examined the roles of avian disease and predation by the introduced brown tree snake (Boiga irregularis) in causing the decline.To identify the occurrence and prevalence of infectious diseases, domestic, native, and introduced birds were sampled for bacteria, viruses, and parasites. No viruses were isolated, and bacteriological tests disclosed no consistent pathogens. No haematozoans were observed in blood slides, and necropsy of 105 carcasses revealed no evidence suggesting an enzootic involving the species examined.A sentinel study was conducted to determine if pathogenic disease agents were present in the habitat of remaining native birds. Disease-free birds from the U.S. mainland, and bridled white-eyes (Zosterops c. saypani) from the island of Saipan served as sentinels. One hundred and six birds were exposed to disease vectors, and 80 were protected. Approximately one-third of the birds were injected with dexamethasone, an immunosuppressant. There was no significant difference in mortality between protected and nonprotected sentinels. Many dexamethasone-treated birds succumbed to a variety of causes, but no consistent significant diseases were detected.The range expansion of Boiga irregularis, the only known avian predator unique to Guam among the local islands, correlated with the range contraction of the forest avifauna. Transects with bird-baited funnel traps revealed high predation by snakes in areas where avian populations had declined. In addition to birds, necropsies showed that B. irregularis consumes small mammals and lizards. A sample of areas trapped prior to snake invasion were re-trapped, and field habitats showed a 94 percent decrease in numbers of small mammals. However, small mammals were abundant in savanna habitat where snakes were rare. By being a generalist vertebrate predator and including the abundant small reptiles as a major component in its diet, Boiga can maintain relatively high densities in forest and second-growth habitats while decimating vulnerable prey species.No significant diseases capable of causing the avian decline were found, and all evidence supported the hypothesis that Boiga is the cause. This is the first documented case of a snake causing the virtual extinction of an insular forest avifauna.